A Feature at z ∼ 3.2 in the Evolution of the Lyα Forest Optical Depth

The effective optical depth in the Lyα forest region of 1061 low-resolution QSO spectra drawn from the SDSS database decreases with decreasing redshift over the range 2.5 ≤ z ≤ 4. Although the evolution is relatively smooth, τeff ∝ (1 + z)3.8±0.2, at z ∼ 3.2 the effective optical depth decreases suddenly by about 10% with respect to this smoother evolution. It climbs back to the original smooth scaling again by z ∼ 2.9. We describe two techniques, one of which is new, for quantifying this evolution that give consistent results. A variety of tests show that the feature is not likely to be a consequence of how the QSO sample was selected nor the result of flux calibration or other systematic effects. Other authors have argued that, at this same epoch, the temperature of the IGM also shows a departure from an otherwise smooth decrease with time. These features in the evolution of the temperature and the optical depth are signatures of the reionization of He II.